PROJECT SUMMARY/ABSTRACT In atrial myocytes excitation-contraction coupling (ECC) and Ca release from the sarcoplasmic reticulum (SR) have unique features that result from the lack or the irregular organization of the transverse tubule membrane system. Atrial myocytes have two types of SR, junctional (j-SR) and non-junctional (nj-SR). Ca release from j- SR is controlled by Ca entry through voltage-gated L-type Ca channels whereas release from nj-SR occurs by subsequent propagating Ca-induced Ca release (CICR). In atrial ECC a fundamental question has remained unanswered: The cardiac ryanodine receptor (RyR) SR Ca release channel has an inherently low cytosolic Ca- sensitivity resulting in a conundrum how CICR from the nj-SR can even be activated. This investigation aims to establish a novel comprehensive model of atrial ECC. In heart failure (HF) the heart undergoes structural and functional changes (cardiac remodeling) that are aimed towards maintaining an adequate cardiac output. We will investigate how at different stages during the development of HF, remodeling of the atria leads to profound changes in atrial Ca signaling, ECC and inotropy that contribute to maintaining cardiac output. Specific aim 1. Determine the unique properties of Ca release in atrial myocytes that ensure robust CICR. We will test a novel hypothesis of a 'fire-diffuse-uptake-fire' (FDUF) paradigm for atrial Ca release and ECC. By this mechanism the coordinated action of RyR regulation by cytosolic and luminal Ca (tandem RyR activation) at the level of individual SR Ca release units as well as the entire SR network, Ca uptake by sarco/endoplasmic reticulum Ca ATPase and intra-SR Ca diffusion assure robust and efficient CICR. Specific aim 2: Determine how atrial remodeling of ECC and Ca release during the progression of HF optimizes cardiac output. We will test the hypothesis that at different stages of HF, stage-specific atrial remodeling determines cardiac output by altering molecular mechanisms that regulate atrial SR Ca release, ECC and contractility. The proposed studies will involve molecular, cellular, intact organ and in-vivo whole animal experiments. We will use a multitude of experimental techniques: high resolution [Ca]i, [Ca]mito and [Ca]SR confocal microscopy, cell shortening measurements, whole-cell voltage and current clamp techniques, single RyR channel recordings, subcellular photolysis of caged compounds, adenoviral gene-transfer, immunohistochemistry and biochemistry techniques. Experimental work is paralleled by computational modeling of Ca release and ECC. A central role plays a rabbit chronic HF model that recapitulates the progression of HF in humans. Rabbit experiments will be complemented with studies on transgenic animals with specific alterations in expression levels of Ca handling proteins crucial to CICR and ECC. Cellular studies will be paralleled with intact heart hemodynamic studies and in-vivo echocardiographic studies in animals during progression of HF.